Martin Luther King (MLK) was a seminary graduate and used biblical imagery and allusions prolifically in his civil rights speeches—as you probably already know. In any case, as you are also probably aware, the phrase from Isaiah 40:5 and is used in the context of his famous “I Have a Dream” speech: “And the glory of the LORD shall be revealed, and all flesh shall see it together: for the mouth of the LORD hath spoken it.” The word ‘flesh’ in this context is translated from the Hebrew בָּשָׂר (basar) meaning, not surprisingly, flesh of humans or animals (Davidson 123). So, I don’t think a simple etymology offers any particularly keen or deeper insight into MLK’s “I Have a Dream” speech. It is just an archaic way of saying humankind (and you can include animals for even more purchase). Rhetorically, archaic language is sometimes used to add authoritative register to a speech enhancing a speaker’s ethos. Even John F. Kennedy used it occasionally; but not as richly as MLK. On another note, King is using the phrase as messianic imagery, recognizing that, ultimately, it probably will not be until the return of Christ that peace will be restored upon the earth. So, this is essentially the meaning in context of the 1963 speech given in Washington, D.C. But knowing King, he chose these words deliberately and there is more to be seen here.
To unpack it further you want to examine the usage of the word in its sitzimleben (“setting in life”)—that is to say the biblical/historical context. Here you have a rich discussion which might add deeper meaning to the context of MLK’s speech. Since the phrase is from Isaiah 40:5, this is our biblical/historical context and we ask, “What do these words mean in the context of Isaiah?” Isaiah was arguably the most prolific prophet of Israel—his book being the longest among the prophetical books of the Old Testament divided into 66 chapters. He wrote at a time when Israel was nearing its demise—being decimated by the Assyrians and Babylonians respectively. Isaiah himself was eventually taken captive by the Babylonians. Much of his book is a call to repentance so Israel would not be wiped-out by her enemies. After many chapters of chastising Israel for its sin, finally, in chapter 40 Isaiah begins offering some consolation prophesying eventual deliverance from exile and peace at some future date. Scholars would say this portion of scripture is especially messianic in nature. Of course, MLK would have known this. King is looking to the future—looking to better times. I might be taking some liberties here, but it is striking to note that “all flesh” has a potent register to it when considering the context of the civil rights movement—it is, arguably, about flesh—black and white, as it were. The notion of “all flesh” means we are all in this together which resonates in the historical context of the book of Isaiah (see Young 3: 31) and the civil rights movement of the 1960’s. We might be different colors, but still people of the same “flesh”—as such, we need to celebrate our common humanity—and realize the “true meaning of [our] creed . . .’that all men are created equal.’”
Davidson, Benjamin. The Analytical Hebrew and Chaldee Lexicon. Henrickson Publishers, 1981.
The Holy Bible, (King James Version). Thomas Nelson Publishers, 1972.
Young, Edward J. The Book of Isaiah (3 Volumes). Wm. B. Eerdmans Publishing Company, Grand Rapids, 1981.